I'm looking for someone that sells velocity stacks that would fit a ZDZ-80 carb. It's the same bolt pattern as the Tillotson and Wallbro carbs used on the 3W-100 and Brison 6.4 engines. I'm looking for a stack with a nice bellmouth made of aluminum. Anyone have any ideas?

OK.... I'm curious,why?

I reckon that it might be worth a couple of hundred rpm if adjusted for proper tuned length. A velocity stack smooths the flow of air entering the carb and can also provide a stronger "sonic" ram effect. Not practical on engines with the carb sticking out the side of the cylinder, but fine for the rear carb motors. I calculate a 7 inch length intake track should tune it for a boost at 6500 rpm.

B&B Specialties makes a Velocity stack that should fit but it doesn't have a bell mouth, the air end is like a tube that has been cut off at an angle. Should work fine though.

Hello there. If you really want one. Please e-mail me. I might be able to help. Thanks Ken

Sounds interesting.If Diablo is right,Mr.Machinest Man might be getting a whole lot of business.

Diablo

How are you arriving at your calculated 7" intake tract?

I assume these carburetors are the same size as the marine guys stuff. Try Bruce Hanson, Warehouse Hobbies, RC International/Cobra engines and I am sure there are others. I have a velocity stack installed on a Hanson modified Zenoah G23H in an XCell Gasser, I got it from Warehouse Hobbies.

A buddy of mine made me a 2" long stack for my zdz-60. I improved 200 rpm. I have room to go longer. how do you arrive at 7" at 6500rpm to give you a boost?
I just assumed the increase was from smoothing the airflow to the carb and not having anything spit out.
Whatcha working on there diablo?

I've got a simple formula that was developed by engineers at Chrysler in the 1950s when they were working on ram induction sonic tuning. Anybody remember those big long ram tubes that cross over from one bank of cylinders to the other? Anyway:

L=72C/N

L=length of intake tract in inches
N= engine speed in rpm where maximum ram effect occurs
C=velocity of wave in ft./sec.

A good number for C is 1100 ft/sec.

However, this formula was developed using 4-stroke passenger V8 engines with relatively mild cam timing. A guess for intake timing is 265 degrees duration. For a 2-stroke, the intake duration is around 165-200 degrees, depending on the particular motor. So I modified the formula to compensate for the shorter intake duration:

L=72C(intake duration)/N(265)

When you work the math for 165 degrees of intake duration and 6500 rpm, the length comes out to 7.59 inches. This length is the total length of the intake tract from the end of the velocity stack to the face of the intake valve (reed, rotary, piston port). Using telescoping tubes and a tach would be an easy way to adjust the optimum for your motor. Just like setting a tuned pipe length - only the other end.

How it works. For those who care, when the intake valve opens a negative pressure wave travels up the intake until it hits the end of the velocity stack. The negative pressure wave reverses when it hits an expanding cone or opening and becomes a positive pressure wave traveling back down the intake towards the valve. If the positive pressure wave is timed to hit the valve just before it closes, then more mixture gets crammed into the engine, and you get a boost in torque. That's why the intake tract length has to be optimized for the engine rpm where you want the boost and also for the amount of intake duration.

Theory sounds good, but when chry,gm and ford had the long intake runners the carb was mounted on the end. You had a carb then the long intake runner then the heads. It worked as the Ram Chargers proved back in the 50's and 60's.(smaller cfm carbs) The fuel charge had to have a high velocity to really work good. It's the same effect as todays tunnel ram intakes on hi performance cars. The carbs have to have low cfm ratings to work on the street. A 350 chevy with 11-1 compression, a decent cam and a set of big ole holly 850s runs like crap until about 5000rpm. then it hauls butt. Same eng with a set of 350cfm carbs can be street driven.

Do you plan on putting the 7" extension between the head and carb or just as a velocity stack on the end of the carb? Imo the biggest advantage would be between the carb and head. and a nice velocity stack to help inprove the flow into the carb. but i think the eng would be a dog until about 5000rpm then it would probably rip the wings off.
Great thread! I like thinking out of the box.

Getting a pressure in syn on a two stroke is the HEART of why two strokes can perform so well.
The tuned two stroke is really done, starting with the exhaust tune.
I have played a fair bit with the 80 single and 40/60 singles , trying to find the smoothest tuning setup.
Not the peak power setup.
I have a ZDZ prototype intake bell-you can see a picture of one on the RE 40 on the Amelung site.
These are not yet released for sale.
Mine also has a coarse wire screen over the mouth.
As soon as my RE 40 arrives , I will see how much this bell helps.
Basically , I see it as a flow straightener-which is a good idea.
The only problem is that on the smaller bore carbs, the choke plate and bar is a big blob -in the way - so - I may remove the choke assy, clean up the carb and put a flapper valve on the bell for choking. this works well and is common use on some OS four strokes.
So far I have found that a tuned pipe-such as the K&S 80 can- with a 11-12" header is the easiest setup for most to use.
I have tried the full length tuned pipes also - any gain there is not worth talking about .
A big version of the VTech tuned can works great -but noisy and not as smooth.
o -once I get a tuned setup on my 40 in my new Hee Haww model ( the Clipped wing Taylorcraft), I will see if the intake bell setup helps that one.
It shouldn't hurt--
We have two 80's running with a tuned can setup from Europe which is discontinued (Damn!) similiar to the K&S and again using the 11-12" header.
We also have a Walbro carb on both and the performance is simply heartwarming.
These two engines are both turning 27x10 MenzS props at 6200. this and no midrange step- in fact you can NOT detect any transfer throughout the entire powerband.
We are setting up another one to see if this is a fluke,luck or whatever.
I looked at my ZDZ "venturi" and noted it seems to be spun from soft aluminum and then peaned neatly into a step in the flange.
It could be whipped out of a billet using a cnc machine -but the equipment used now to spin -such as the D Brown spinners would also do a great job. I would ask Dave -but it may not be a very high volume item--
I will let you know if the venturi I have is worth further research.
Please let me know how yous works -
You will probably find that tuning the exhaust THEN adding the flow straightener/ venturi/ tuned intake runner whatever it turns out to be -- is the best procedure.

Thanks Dick for your thoughts. When I first saw the ZDZ-40 RE with the velocity stack, a bell went off in my head. I was hoping that ZDZ would start selling the stacks as an option. I also found out that 3W has a velocity stack for the 3W-100 on Aircraft International's website that should also bolt up to the ZDZ-80. They get $17 for it. I plan on putting the stack on the carb and maybe adding a section to it to get the desired length. You're right that spinning it out of aluminum would be a cheap way to make it. Actually you could draw it from a flat plate in a die, but the tooling would cost more. Certainly it could be CNC machined, but you'd be wasting a lot of metal. Some of the older guys might remember V8 Chevy racing motors with Weber carbs, a single venturi and long intake tube for each cylinder with the stack on top. Same thing all over again. I'm sure you are correct about the exhaust system being critical to the power, but for my H9 Cap I'm just going to use the J&A Pitts, purely for convenience. I've got my buddy Hyper Tom convinced on the need for a K&S system for his new Aeroworks 31% Extra, and EJB is building the plane for him. Tom needs a quiet plane for Imac.

Another random thought - Have you tried modifying the intake rotary valve timing? Many years ago Kawasaki made a hop up kit for their Bighorn that used a longer opening for the rotary valve, along with a pipe and carb jets. I know we're not after HP at high rpm so maybe this isn't the way to go with longer timing.

Which model of Walbro carb have you got on the 80? There's a wealth of info on carbs from a fellow named EC Birt. He has a lot of experience with Tillotson, Walbro and a few other makes of carbs. Check out www.eccarburetors.com If you are getting better results from a different brand of carb, this fellow probably has the knowledge to explain why.

I will surely grab one of the 80 bells! Thanks for the tip.
I tried a 7 degree increased opening - no improvement -I gave it away - I got it on a trade engine -from Randy Villines.
The engine was fine -but changing back to a stock opening proved the best setup.
I always am puzzled by the desires to change openings etc-- these are the easiest changes for the manufacturere to make -so why didn't he see the advantage.
One these two strokers the BIG advantage is a tuned system which matches the engine design.
when done right - the exhaust tuning also creates maximum pull at the intake -
I have a Pitts muffler on my Bucker but justfor the smoke novelty.
It is a noisy thing.
I know about birt- the carbs I am using are the WJ 38A series - these have a great cold start /idle response -yet a perfect transition.
All of these carbs have different discharge hole setups which respond to different vacuum/throttle blade positions.
the "signal" from a particular engine porting setup may be weaker or stronger - and the carbs selected will be good -or flat or loadup etc..in lowto high speed transfer of the fuel discharge.
I learned this crap when I did a strong 274" flathead Ford - in 1955. it had 3 Stromber 48's
When I went to Chevs in 1958- I again used these carbs on a 3 carb manifold and consistantly blew off dual/single four barrel setups.(a 283)
Our D Gas car ran over 100 -in 13's.it was also the daily driver.
I learned then the value of long flat constant powerband- we ran the quarter in basically two gears.
Today that is a "nothing" race setup-the goal is max output .and a unusable car.
the smooth powerband for our aerobatic mosdels tho , is still where it's at.

Some of the weedie conversion people are putting velocity stacks on the side-carbed engines (e.g. Homelite), supposedly to minimized the pressure reduction caused by the venturi effect of the incoming airstream.

http://www.carrprecision.com/Pages/prod02.htm

I don't think these should be called "velocity stacks" since they don't function per the namesake. Intake tuning is akin to exhaust tuning. The difference is that whereas exhaust is tuning for negative pressure to optimize scavenging, intake is tuning for positive pressure to maximize intake charge filling. These days, most auto engines are using a resonant intake chamber to achieve the same goal but with a wider powerband. This is why those who replace the OEM airbox in their cars with a supposedly less restrictive cone filter often end up with a net power loss.

the conversion guys and those who run "up front carbs" on model engines both have the same potential problem.
The airstream passing from front to rear over the carb can raise hell IN the inlet .
lengthening the inlet (adding the "venturi")will sometimes move the airflow problem.
angling the mouth of the "venturi" will sometimes help.
The air signal to the diaphragm is usually, also a culprit in these setups.

the carb's original design, which is to use the signal from the crankcase for fuel delivery, gets all fu--- up when the airstream on the inlet and the diaphrabm is a variable.
These "patches" are referred to as many things-but venturis- they are not.
The old term velocity stack is better -FWIW.
The car guys who have access to an on board computer tuning program can benifit from low restriction inlets -but as you mentioned - this is not a great idea all by it's self

Hey Diablo,
Ya think one of these is worth a try on Hyper Toms' 3w 80 twin?
(Tom has a 3w 80 twin on a Aeroworks 33% Edge that starts to run out of steam on extended verticals on hot days)

Well I see on the 3W site Aircraft International that they offer a carbon fiber intake manifold (actually it's a banana shaped velocity stack) that fits inside the cowl on the bottom-carb motors. They hook the diaphragm metering tube up at the elbow of the stack. It might be good for some more rpm as well as fixing the mixture problems in the midrange that some (?) of the 3W twins have. Only (?) $40. Let's talk Tommy into spending his money.q;}

And just what I was looking for. My 3W 80 shows the symptoms of a rich mid range, and an engine rhythm/spped change when I lay it on it's side. I just don't think it's in the needles. Has anyone tried the mods where you add a brass tube to the carb and vent it in the fuse? This is my first gasser and I love it, except for this. Just can't get into a good rhythm
I would like your opinions and fixes_Bob

http://www.rcuniverse.com/showthread...136&forumid=37]

Hey Bob:
Yes, I've tried the carb metering diaphragm vented to the fuse with a tube. On my 3W-100, it still goes rich in rolling circles and slow point rolls - drives me nuts. I bought a carbon fiber elbow to try out, but haven't put it on yet.

Check out the velocity stacks at Brison Aircraft. I was impressed, very nice velocity stack. If you want a picture email me.

Man, I'm not happy to hear that. I just completed the same procedure on my 80. If it acts the same I'm gonna be way sad
I had a long talk with Jim Cline the other day. I really like that old man. He'll jibber jabber as long as you'll listen, but he makes sense. If running the tube in the fuse doesn't help, he recommended trying the tube at the inlet where the choke is. His first choice though is in the fuse. I couldn't get Gerhardt in time to have him send me one of the intakes. He was out the door to Toledo and couldn't ship. Same iwth Cactus. One way or the other we have to be able to work it out___RIGHT

RC Showcase now has trumpet shaped velocity stacks in stock for all ZDZ engines. Not on our website yet. Call Mike Dooley at RC Showcase for pricing. 301-374-2197

David Garrison
RC Showcase
http://www.rcshowcase.com

I too tried the vented tube in the fuse to no avail. I then tried a makeshift tube to the choke area with not much better results. I'm ordering the elbow tomorrow. What do I do if that doesn't help. I thought these gassers weren't supposed to have the inherent miseries of the little glo engine

I have been having the problem of my ZDZ-80 going rich in a slow roll.
My ZDZ came with a Bing carb. Do you fellows find that a Walbro is better?